This is an older tutorial I made that I decided to upload.
It is for BEGINNERS in shading and with a very simplified explanation of cel shading. I use Photoshop here and some basic layering.
Even though the art is old and outdated I hope this message still stands true for many young artists: don’t hold back and be scared to shade! You will mess up (like I did) and it’s okay! Just keep practicing and experimenting!